Everybody's needs are different. For some 128GB will be more than enough, for others, not even 256GB will be enough. And this doesn't matter if it's a primary or secondary system....
The only thing holding me back is the price. You can easily find 256GB SSD's for $~160 but the thought of paying $200 just to get a 128GB upgrade is really putting me off.
Has anyone regretted getting the 128GB?
I just ordered the 128 GB model and will be "downgrading" from my 256 (current) SSD to it. For my needs, the extra space is irrelevant, and most of my large files (i.e. media) is all on external devices anyway, so putting that $200 towards something else is better.
It really depends on the needs of the user though. Personally speaking, I don't think 256 is sufficient for the "average" user either, assuming that don't stream all of their media (which is increasingly becoming the case), but that's just me. 256 is good for those who boot camp (for gaming) though, and for those with multiple VMs, assuming the OS is large.
I got the 128 and picked up ad 256 chip for $150. I sold the 128 for $90.
Where did you buy the upgrade?
I went 8/256 against my better budgeting thoughts, but I don't want to have any worries as I plan to keep it for 4 years.